hard to say, try an watch them at night with a dim light on, as both cats will be most active at night, maybe try an divide one at a time an wait to see. from what i found from both cats an more so the sun cat is that while other day active fish sleep some will go closer to bottom or stay in one spot, an when the cats are very active at night searching for food they will attemp to eat anything even if they realize its to big or they dont want it,, from what i seen it looks like they bite to see if they will wanna eat it lol. once u figure out who is doing it might need to get rid of the cat that is or the fish being bit. but if u keep the cats be aware that it might happen to other fish. how bad is the damage, could be minor. keep a close eye, as those cats will grow very fast an it might go from nibble to dinner. good luck